The introduction of an organic semiconductor makes it possible to realize a flexible display that can be manufactured on a flexible plastic substrate. Since a result of research showing that polyacetylene as a simple conjugate polymer has semiconductor properties and if polyacetylene is doped, it may have electrical conductivity of metal was reported, research and development on organic semiconductor materials, especially organic light emitting diodes, has been in progress and various mobile electronic devices employing organic light emitting diodes have been commonly used. Further, it has been tried to manufacture a driving mechanism for such organic light emitting diodes with organic materials, and, thus, technology development on organic thin film transistors has been in progress.
A configuration and a principle of operation of the organic thin film transistors are basically the same as those of metal oxide semiconductor field effect transistors (MOSFET) using amorphous silicon or polycrystalline silicon. However, the biggest difference between the organic thin film transistor and the MOSFET is using an organic semiconductor as a semiconductor material instead of using silicon. Even a few years ago, an organic thin film transistor using an organic semiconductor as an active layer semiconductor material, a glass substrate or a silicon wafer, having very low, electrical resistance as a substrate, and metal and an inorganic oxide film as an electrode and an insulating layer, respectively, has been researched. However, recently, research on an organic thin film transistor (TFT) using polymer materials for a substrate, an insulating layer and an electrode has been in progress, and, thus, a public use of a flexible display has been accelerated.
In addition to this, development of an organic semiconductor is important in developing organic solar cells. Solar cells mainly feature p-n junction solar cells using silicon, but improvement in a performance and price competitiveness of the solar cells may reach the limit. Therefore, development of new solar cells which can overcome limitations of conventional solar cells is needed. There have been suggested various solar cells over silicon solar cells, and an organic solar cell capable of remarkably improving an economic feasibility of a silicon solar cell has been regarded as having greatest potential. The most important thing in developing organic solar cells is development of a light absorbing layer that absorbs lights and development of an n-type semiconductor layer and a p-type semiconductor layer that receive electron holes and electrons generated when the light absorbing layer absorbs lights. Various organic polymer have been developed as a p-type semiconductor, and, thus, it is possible to make a selection in various ways, but there has not been suggested a particular system having a distinctively excellence as an n-type semiconductor. In a dye-sensitized solar cell, TiO2 has been used to form an n-type semiconductor and a light sensitizer using ruthenium organic metal compound has been used. However, such a technology has been technically protected and is expensive. Eventually, the development of the organic semiconductor is most important in developing organic solar cells having high efficiency and price competitiveness.
It is possible to reduce manufacturing cost with a semiconductor device and a solar cell using an organic semiconductor as compared with a conventional semiconductor device using an inorganic semiconductor such as Si, and the semiconductor device and solar cell using the organic semiconductor have been expected to have flexibility. Various organic semiconductor compositions such as polythiophene and rubrene have been researched, and it has been reported that a transistor including a channel forming area formed of such organic semiconductor compositions has mobility equal to or similar to mobility of a transistor including a channel forming area formed of amorphous silicon (see, for example, APL Vol. 80, No. 6, 1088-1090 (2002)).
A number of organic semiconductor compositions such as pentacene or conductive polymers having high mobility of electron holes have been known as p-type organic semiconductors used for organic TFTs, organic light emitting diodes, solar cells and the like.
However, if a channel forming area is formed of these organic semiconductor compositions, the organic semiconductor compositions are very slightly soluble in organic solvents and it is difficult to apply a coating process to these compositions, and, thus, a film is formed by vacuum deposition. These organic semiconductor compositions may have affinity with organic solvents by introducing alkyl chains or other substituents, so that these organic semiconductor compositions can be dissolved in the organic solvents. To be specific, it has been reported that poly-3-hexylthiophene (P3HT) formed by introducing a hexyl group to polythiophene is dissolved in organic solvents such as chloroform or toluene and a channel forming area is formed through a coating process such as a spin coating process (see, for example, APL 69(26) 4108-4100 (1996)).
Meanwhile, a polyacene compound as a condensed polycyclic compound is a molecule having a n-electron conjugated system in the same manner as polyacetylene or polyphenylene, and theoretically, the polyacene compound has a small band gap and has been expected to serve as an organic semiconductor composition having outstanding functions as compared with polyacetylene. A substituent introduced to the polyacene compound can be used for molecular binding or binding with a functional group of an insulating film surface and can be used to control a distance, a position and arrangement of an acene structure or can be used for patterning. The polyacene compound has benzene rings linearly connected to each other, and the polyacene compound which does not contain a substituent has a property of being slightly soluble in organic solvents as the number of benzene rings increases. Particularly, a polyacene compound having five or more linearly-fused benzene rings, such as pentacene, is not dissolved in most organic solvents and it is very difficult to form a uniform film through a spin coating process, and even if possible, an organic solvent and a temperature are very limited (for example, trichlorobenzene, 60° C.-180° C.).
Further, it has been widely known that as the number of benzene rings increases, the polyacene compound has low stability and particularly, pentacene has low resistance to oxidation and can be oxidized by oxygen in the air. As an example of introducing a substituent to a polyacene compound, 2,3,9,10-tetramethyl pentacene has been reported (see Wudl and Bao, Adv. Mater. Vol. 15, No. 3(1090-1093), 2003). However, 2,3,9,10-tetramethyl pentacene can be slightly dissolved in slightly heated o-dichlorobenzene and in reality, a channel forming area constituting a field effect transistor is formed by vacuum deposition.
It is also described in Japanese Patent Laid-open Publication No. 2004-256532 that 2,3,9,10-tetramethyl pentacene or 2,3-dimethyl pentacene is dissolved in o-dichlorobenzene. However, they are dissolved at temperature of 120° C. and it is not described that they can be dissolved at room temperature.
That is, a polyacene organic semiconductor compound such as pentacene needs a high production cost and can be sublimated under a depressurized atmosphere at 300° C. and decomposed in the air at a temperature higher than 300° C., and also, the polyacene organic semiconductor compound has low solubility to aqueous solvents and organic solvents, so that it is difficult to apply a liquid phase process to the polyacene organic semiconductor compound and instead, a gas phase process is applied to the polyacene organic semiconductor compound. Accordingly, a new organic semiconductor compound capable of solving the above-described problems has been demanded.